The present invention relates generally to an apparatus for writing information to an optical recording medium, like an optical disk, by a mark edge method, more particularly, the present invention relates to an optical recording/reproducing apparatus that can record a mark on a recording medium by irradiating the recording medium with a pulsed laser radiation beam and read the information stored on the recording medium while maintaining the proper definition of, or without narrowing the middle portion of, the mark even if the recording medium exhibits a low recording sensitivity.
Examples of known recording mediums storing optically rewritable information thereon include phase-change storage media and magneto-optical recording media. In writing information onto a phase-change storage medium, an information layer of the medium is irradiated with a focused laser beam, thereby partially heating and fusing the information layer. The highest temperature the information layer can reach due to the heat applied thereto or the cooling process of the layer differs depending on the intensity of the laser radiation incident thereto. Thus, the optical characteristics of the information layer, such as the refractive index thereof, are locally modifiable by modulating the intensity of the laser radiation emitted. More specifically, if the intensity of the laser radiation is higher than a predetermined reference level, part of the information layer of the recording medium that has been irradiated with the radiation is rapidly cooled from an elevated temperature so as to be amorphized. If the intensity of the laser radiation is relatively low on the other hand, the irradiated part of the information layer of the recording medium is gradually cooled from an intermediate to high temperature and therefore crystallized. The amorphized part of the information layer of the recording medium is called a xe2x80x9cmarkxe2x80x9d, while the crystallized part is called a xe2x80x9cspacexe2x80x9d. That is to say, the mark and space have mutually different optical characteristics in terms of their refractive indices, for example. Accordingly, binary data is storable in the information layer of the recording medium by arranging the marks and spaces in a specific pattern. As used herein, the laser radiation for use in information recording will be called xe2x80x9cwrite radiationxe2x80x9d.
In reading out information stored on a phase-change storage medium, the information layer thereof is irradiated with a laser radiation beam with an intensity low enough to not cause any phase change in the information layer and a radiation beam, which is reflected from the information layer, is detected. As used herein, the laser radiation for use in information readout will be called xe2x80x9creadout radiationxe2x80x9d. The mark, or the amorphized part of the information layer of the recording medium, has a relatively low reflectance, while the space, or the crystallized part of the information layer of the recording medium, has a relatively high reflectance. Accordingly, by recognizing the difference in the amount of the radiation reflected from the mark and space, a read signal can be obtained.
Information can be recorded on such a recording medium by a pulse position modulation (PPM) or pulse width modulation (PWM) technique. A recording technique which uses PWM is also called a xe2x80x9cmark edge recordingxe2x80x9d technique.
According to the PPM recording technique, marks are recorded with the space between the marks varied, and information to be written is assigned to positions of the marks. Each of these marks is represented as a pulse with a relatively short, constant pulse width. In contrast, according to the PWM technique, marks of various lengths are recorded with the space between the marks also varied, and information to be written is represented by edge positions of the marks and spaces with a variety of lengths. Generally speaking, the density of the information recorded can be higher with the PWM technique than with the PPM technique.
In performing a PWM recording, longer marks are recorded compared to the PPM recording. If long marks are recorded on a phase-change storage medium, however, the widths of those marks might be non-uniform, because the information layers of media of this type may accumulate and dissipate heat in various manners and their recording sensitivities may be greatly different from each other. It is also known that if the information layer is continuously irradiated with radiation for a long time to record a long mark therein, then the second half of the long mark is likely to increase its width because too much heat is accumulated in that part. To avoid such an unfavorable increase in mark width, a write strategy, by which the radiation is irradiated for recording purposes as a greater number of pulses each with an even short width, was adopted. Methods and apparatuses of this type, that is to say, multi-pulse mark recording methods and apparatuses, are disclosed in U.S. Pat. Nos. 5,490,126 and 5,636,194, for example.
FIG. 2(a), FIG. 2(b), FIG. 2(c) and FIG. 2(d) illustrate waveforms of write radiation, shapes of marks formed in the information layer, waveforms of read signals and binary data obtained by digitizing the read signals, respectively. It should be noted that the waveform of write radiation is defined by the waveform of an electrical signal used for modulating the write radiation. As used herein, the electrical signal will be regarded as a collection of xe2x80x9cwrite pulsesxe2x80x9d. The power of the write radiation (hereinafter, simply referred to as xe2x80x9cwrite powerxe2x80x9d) is proportional to the amplitude of a write pulse. Depending on the type of a radiation source (e.g., semiconductor laser diode), a non-essential difference may be found between the waveform of write radiation and the waveform made up of write pulses. In this specification, however, the waveforms of the write radiation and write pulses will not be strictly distinguished from each other.
First, referring to FIG. 2(a), the waveform of the write radiation used for forming a single mark consists of a first pulse 1, a multi-pulse train 2 and a second pulse 3, which appear one after another in this order on the time axis. The write power is modulated among peak power Pp, bias power 1 Pb1 and bias power 2 Pb2. It should be noted that although the term xe2x80x9cmulti-pulse trainxe2x80x9d generally means a train made up of at least two pulses, just one pulse located between the first and second pulses will also be labeled as such in this specification for convenience sake.
In an interval during which a single mark is being formed in the information layer by irradiating the information layer of the recording medium with the write radiation, the write power is modulated between the peak power Pp and the bias power 2 Pb2. As used herein, this interval will be called a xe2x80x9cmarking periodxe2x80x9d. On the other hand, in an interval during which a single space is being formed in the information layer of the recording medium, the write power is maintained at the bias power 1 Pb1. As used herein, this interval will be called a xe2x80x9cspacing periodxe2x80x9d.
In general, an optical recording/reproducing apparatus has to write or read information appropriately onto/from an optical information carrier with various recording properties. Thus, if information is written on an information carrier with a relatively low recording sensitivity while keeping an average write power (i.e., an average of the write power during the marking period) constant, then the lengths and widths of marks formed in such a carrier tend to be smaller. Accordingly, after having initialized the write power of a radiation source at an appropriate value while taking the recording sensitivity of an information carrier into account, a conventional optical recording/reproducing apparatus compensates for the write power to adaptively change the lengths and widths of marks to be formed. This process is called xe2x80x9cwrite power learningxe2x80x9d. More specifically, such an optical recording/reproducing apparatus compensates for the write power by recording a relatively short mark on the information carrier for testing purposes and then modulating the write power such that the short mark can be recorded accurately. This strategy has been adopted because it has been more important than anything else to record a short mark resulting in a read signal with small amplitude.
However, a read error is still unavoidable even if the write power is compensated for by the conventional technique. Also, a relatively long mark is more likely to cause such a read error.
An exemplary mark 4 is illustrated on the left-hand side of FIG. 2(b). Such a mark 4 is formed if the thermal energy (or the average power applied by the write radiation during the marking period) associated with the multi-pulse train 2 is less than a minimum required level. As shown in FIG. 2(b), the mark 4 is relatively wide at its front and rear edges but is relatively narrow in its middle portion between the edges. A to mark recorded by the conventional technique results in this unfavorable phenomenon, hereafter referred to as xe2x80x9cmiddle narrowingxe2x80x9d.
If such a mark 4 is irradiated with readout radiation, and the radiation reflected from the mark 4 is typically received at a photodetector and converted into an electrical is signal, then a read signal 5 with twin peaks is obtained as illustrated on the left-hand side of FIG. 2(c). And if the read signal 5 is digitized with respect to its threshold value 6, then two discrete pulses 7 and 8 are formed as illustrated on the left-hand side of FIG. 2(d). As a result, neither the precise locations of the edges of the mark 4 nor the length of the mark 4 can be recognized correctly, thus causing an error in reading the recorded data from the recording medium.
The middle portion of a mark, i.e., part of a mark located between its front and rear edges, where the level of the associated read signal is relatively low and which will be erroneously recognized as a xe2x80x9cspacexe2x80x9d, not xe2x80x9cpart of the markxe2x80x9d, when the read signal is digitized, will be hereafter referred to as a xe2x80x9cread-error-inducing portionxe2x80x9d.
Even in the conventional optical recording/reproducing apparatus, if an increase in number of read errors is sensed by a system controller in the apparatus during the process of compensating for the write power, then the write power is automatically adjusted in such a manner as to reduce the read errors. The conventional compensation technique is illustrated on the right-hand side of FIG. 2(a)-(d). According to the conventional write power compensation technique, the power level of each pulse in the write radiation is increased by the factor of xcex1 (where xcex1 greater than 1), thereby irradiating an optical information carrier with the write radiation with the waveform shown on the right-hand side of FIG. 2(a), where Ppxe2x80x2=xcex1xc3x97Pp, Pb1xe2x80x2=xcex1xc3x97Pb1 and Pb2xe2x80x2=xcex1xc3x97Pb2. However, if all of these three power levels are increased by the same factor, then a resultant mark 10 will be much longer and wider than a desired mark 9 as shown on the right-hand side of FIG. 2(b). Thus, such an excessively long and wide mark 10 will result in a read signal 12 with a waveform laterally expanded compared to a desired read signal 11 as shown on the right-hand side of FIG. 2(c). And if that read signal 12 is digitized with respect to its threshold value 6, then a mark length 14, which is represented by the width of a pulse of the binary data obtained, is longer than its appropriate length 13 as illustrated on the right-hand side of FIG. 2(d). As a result, neither the locations of the edges of the mark 9 nor the length of the mark 9 can be recognized correctly, thus also causing a read error.
It should be noted that such a problem is not unique to a phase-change storage medium but might happen to any other optical information carrier, e.g., a magneto-optical recording medium.
The invention provides an optical recording/reproducing apparatus that can further improve the conventional write power compensation, accurately determine whether or not a relatively long mark will have a narrowed middle portion and correct the write power to form the long mark in its desired shape.
An inventive apparatus forms multiple marks with nine mutually different lengths, which are represented as 3T through 11T according to an eight-to-sixteen modulation coding technique, in an information layer of a rewritable recording medium by irradiating the information layer with a pulsed radiation beam. A test mark with a length equal to or greater than 7T is once recorded in the information layer, a signal associated with the test mark is read out and then an average of power, which is applied to marks with various lengths equal to or greater than 7T, is changed for part of each said mark to uniformize the widths of the marks.
Another inventive apparatus also forms multiple marks, which are associated with information to be written, in an information layer of a recording medium by irradiating the information layer with a pulsed radiation beam such that optical characteristics of the information layer are locally changed. The apparatus includes: information write means for generating write pulses in forming each said mark and for modulating the radiation beam with the write pulses; information readout means for sensing a variation in the optical characteristics of respective parts of the information layer where the marks have been formed; and a long mark data generator for getting at least one test mark written in the information layer by the information write means. The length of the test mark is equal to or greater than a preselected length. The apparatus further includes a mark width comparator for determining whether or not the test mark includes a read-error-inducing portion in which the optical characteristics have not changed sufficiently for reading out a signal associated with the test mark correctly. If there is a read-error-inducing portion in the test mark, an average power of the radiation beam is increased from an initialized level in forming a mark with a length at least equal to that of the test mark, thereby correcting the power in such a manner as to eliminate the read-error-inducing portion from the mark.
In one embodiment of the present invention, the power is preferably corrected by partially increasing the average power of the radiation beam for a middle portion of a mark to be formed.
Still another inventive apparatus also forms multiple marks, which are associated with information to be written, in an information layer of a recording medium by irradiating the information layer with a pulsed radiation beam such that optical characteristics of the information layer are locally changed. The apparatus performs a test recording operation. The operation includes the steps of: a) writing at least one test mark in the information layer, the length of the test mark being equal to or greater than a preselected length; b) sensing a variation in the optical characteristics of part of the information layer where the test mark has been formed; and c) determining whether or not the test mark includes a read-error-inducing portion in which the optical characteristics have not changed sufficiently for reading out a signal associated with the test mark correctly. If it has been determined in the step c) that there is a read-error-inducing portion in the test mark, an average power of the radiation beam is increased from an initialized level in forming a mark with a length at least equal to that of the test mark.
Yet another inventive apparatus forms multiple marks, which are associated with information to be written, in an information layer of a recording medium by irradiating the information layer with a pulsed radiation beam such that optical characteristics of the information layer are locally changed. The apparatus includes: a memory where information defining a pulse train, which will drive a source of the radiation beam, is stored; a write circuit for generating the pulse train in accordance with the information stored on the memory; a read circuit for reading out data associated with the marks that have been formed in the information layer; and a controller, connected to the write and read circuits, for controlling write and read operations. The controller is programmed in such a manner as to perform the write and read operations for a mark with a length equal to or greater than a preselected length and to update the information stored on the memory such that the pulse train is optimized for the mark with the length.